The ultrasonic absorption, in magnetically aligned p-cyanobenzilidine-p-n-octyloxyaniline (CBOOA) in the frequency range 0.8-25 MHz, is studied. Near the nematic-smectic A transition a critical relaxational behaviour is observed. The relaxation time of the order parameter is deduced from these measurements and is in qualitative agreement with the recent calculation by F. Brochard. The frequency dispersion of a friction coefficient is also reported. The data are consistent with the high frequency and zero frequency results. In addition, a confirmation of an intramolecular relaxation process is given